Current electronic circuits often require one or more direct current (DC) supply voltages, and various systems are so designed for power conversion accordingly, including the charge pump. A charge pump is a capacitor and oscillator based circuit which converts a DC input to a DC output which is either higher, lower, or inverted in voltage value. FIG. 1 shows a conceptual diagram of efficiency history when a boost charge pump operable with x1 mode and x1.5 mode is operated, in which curve 10 represents the efficiency of the charge pump operating with x1 mode, and curve 12 represents the efficiency of the charge pump operating with x1.5 mode. Usually a battery is used to provide the power for the charge pump, and the supply voltage provided by the battery will decrease with use of the battery. Once the battery becomes insufficient to provide enough voltage for the output loading, the charge pump will change to an operating mode with higher conversion ratio, for example from x1 mode to x1.5 mode as designated by the trip 14; on the contrary, if the battery voltage rises up again, or the output loading decreases, the charge pump will change back to the previous operating mode which has lower conversion ratio, for example from x1.5 mode to x1 mode as designated by the trip 16. Conventionally, the equivalent resistance of the charge pump is required to be calculated in order to determine when it needs to change the operating mode of the change pump. However, this calculation is usually influenced by many factors such as noise interference, measurement accuracy, and temperature effect, and hence easily has an error which will interferes the determination. For this reason, a hysteresis voltage is added to the control system so as to delay the mode transition timing of the charge pump, in order to avoid a momentary misjudgment on the mode transition. However, if the hysteresis voltage is set higher, the mode transition timing may be delayed so much to cause efficiency degradation of the chip's operation; while if the hysteresis voltage is set lower, it may not be enough to avoid the misjudgment. Generally, the more accurate the equivalent resistance is calculated, the lower the hysteresis voltage can be set; on the contrary, higher hysteresis voltage is required to avoid the misjudgment if the equivalent resistance is calculated less accurate. Nevertheless, to calculate the equivalent resistance more accurate, a more complicated circuit is needed.
Therefore, it is desired a method and circuit for mode transition control of a charge pump, without calculating the accurate equivalent resistance of the charge pump, or using a high hysteresis voltage to avoid the misjudgment on the mode transition.